Sylvie would see freedom - or her idea of it, anyway. She's be someplace tropical surrounded by hot shirtless blokes, alcohol, and no parents in sight.
Marietta would see herself with her face clear of Hermione Granger's hex and nor brain damaged - I'm not sure how the second would manifest, but it would!
Stacey was actually the hardest. I think she'd see herself as a successful grown-up, probably. Happy with a family and a career.